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4 Diodes vs. Bridge rectifier

I notice (among the DIY audio enthusiasts at least) that when it comes time to design a power supply for an amplifier, DAC or whatever that the parts list will inevitably include something like "4 x MUR860 diodes" for the sake of building a full wave bridge rectifier (MUR860 is a particularly popular choice).

However, you get these all-in-one bridge rectifier "chips" that essentially include 4 diodes in the correct bridge configuration, and:

  1. are often housed in metal casings that can be cooled if necessary
  2. can typically handle much higher voltage/current ratings
  3. occupy less physical/PCB space than 4 discrete diodes
  4. often cost less than 4 discrete diodes!

Question: Are there any benefits in using separate diodes over a single bridge rectifier chip, and if not, why does it seem so popular to do so? Is this just about the satisfaction of "making it yourself", or perhaps some audiophoolery at work? Thanks!

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    \$\begingroup\$ Learning how to build a bridge rectifier circuit is popular in school therefore you typically use single diodes to build it. In the real world, you will need to keep your circuit as small as possible while keeping the performance the same. This is why you would need to use a single chip instead of four diodes. \$\endgroup\$ – 12Lappie Aug 11 '17 at 15:34
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    \$\begingroup\$ Perhaps you have seen this article. One guy claims that the MUR860 sounds better than all other diodes and all the other muppets follow. \$\endgroup\$ – Steve G Aug 11 '17 at 15:42
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    \$\begingroup\$ One particular reason often has to do with purchasing. If it's a simple diode, like a 1N4004, your company may buy them by the ton for next to nothing. As such using four of them in place of ordering small quantities of a new component at a larger cost becomes less attractive. Footprint wise, it often makes little difference, and with auto-assembly, labour is not an issue. Further spreading the wattage over four parts often alleviates the need for a heat-sink. \$\endgroup\$ – Trevor_G Aug 11 '17 at 15:44
  • \$\begingroup\$ @Trevor, Since the molded bridge contains typically the same part you would use as a discrete (1N400 say), and the plastic molding is a relatively poor thermal conductor ...the molded bridge is going to be worse thermally than discrete parts. \$\endgroup\$ – Jack Creasey Aug 11 '17 at 15:55
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    \$\begingroup\$ Note that the one-piece bridge rectifier is not a "single chip" (monolithic) -- internally it's four separate diodes mounted on a special lead frame. On the other hand, dual diodes (e.g., in SOT-23 or TO-220 packages) normally are monolithic. \$\endgroup\$ – Dave Tweed Aug 11 '17 at 16:27
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Can't believe I wrote all that crap about diodes...

MUR860 will indeed sound better, but the explanation is a bit subtle:

Silicon diodes do not turn off instantly. As the voltage across the diode goes negative, current still flows in the reverse direction for a short time, until the charges stored inside the diode are cleared out. When this is done, the diode turns off.

Different diodes have wildly different recovery characteristics, as shown in this scope plot:

enter image description here

(source)

The current does indeed go negative (the "wrong" direction for a diode) for a time which is called "recovery time". The red one takes longer.

In a DC-DC converter, it is crucial to have a diode that turns off quickly. Imagine using good old 1N4001, with its recovery time trr=30µs in a DC-DC converter running at 200kHz (cycle time 5µs). It wouldn't even have time to turn off. It wouldn't work at all. This is why DC-DC converters use much faster diodes.

Now, back to your audio stuff. Check the red and purple traces above, you'll notice that the red one takes longer, but turns the current off softly. The purple one turns off very sharply, with huge di/dt (4 Amps in like 10ns). It doesn't happen like this in a 50Hz rectifier, the current doesn't have time to go to amps before the diode turns off, only a few mA. But you get the idea.

Once the diode is off, it is now a capacitor. Whatever inductance is in the traces, wires, etc, around will form a LC tank circuit with it, and ring.

The amoung of ringing depends on the turnoff sharpness, and the current at which turn-off occurs. Fast-soft recovery diodes produce less ringing.

Now, this ringing is usually at a rather high frequency. Also the sharp di/dt at turnoff generates wideband RF noise. This will couple into nearby circuitry, adding all kinds of noise and trash to sensitive signals. This is not audiophoolery, just engineering.

That said, MUR860 is expensive, so you can use cheap diodes with slow crummy recovery, if you put caps across them to absorb the turn-off noise spike. Every mains powered AM/FM tuner does this, as well as most consumer audio equipment. Manufacturers won't put a part in unless it's needed! Everything is cost optimized. But without the caps, the tuner would be overcome by the noise, and not receive the radio.

You can then add a snubber on the transformer secondary to dampen the LC ringing.

Question: Are there any benefits in using separate diodes over a single bridge rectifier chip

Benefit is you can pick fast-soft recovery, or schottky diodes. Canned diode bridges usually consist of ultra-slow diodes.

and if not, why does it seem so popular to do so?

Because it works. Note that 4 caps, at 3 cent each, work just as well, but the bragging factor is less. Fast diodes are sexier and score more snake oil points.

EDIT, an old scope trace from my harddisk... BYV27-150 cheap fast diodes, small 12V 10VA transformer.

Blue is transformer secondary. Flat top part is when diode is on, supply capacitor is charging, limiting voltage on transformer secondary due to its internal winding resistance. Blue trace makes a step down when diode turns off. It's very obvious, it drops by 1V, can't miss it!

enter image description here

Note diode only turns off at the peak of sine wave if the load draws zero current. When the load draws current, which is usually the case, diode turns off after the peak.

Now, I like to watch this through a highpass filter (yellow trace below). Amplitude is attenuated, as highpass filter must use a tiny cap, about 100pF, or else it would snub what I want to observe, so scope input capacitance interacts with it. But the general shape of the signal should be alright. Notice nasty sharp spike followed by HF ringing. Higher Qrr diodes like 1N4001 would be a lot worse.

enter image description here

EDIT 2

I've been restoring an old amp, changing the electrolytics from 1979... and this amp doesn't have caps across the diode bridge. Probably because it doesn't have an AM tuner. Anyway, the way to do this is you stick the scope probe on the insulator of one of the transformer secondary wires. No need to make any kind of contact (except ground the probe obviously) This trash is coupling through the wire's insulation and into the scope probe.

enter image description here

That's a rectifier recovery spike. Unfortunately, it appears as common mode on the transformer wires, which means the whole secondary winding acts as antenna and will capacitively couple the spikes into nearby circuits. High-impedance stuff like the volume pot is a prime victim.

This is probably why this amp has a transformer which is shielded inside a metal can. It would have been cheaper to put caps across the diodes IMO...

enter image description here

Now, of course the secondary voltage can also be measured, by means of sticking the probe on the PCB terminals:

enter image description here

It has the usual look: flat top, then a spike and instant drop down a few volts when the diode turns off. Zooming on the spike:

enter image description here

So, the secondary transformer wires have 22 volts spikes on them (!!!!) with a rather fast risetime of 2µs.

The issue is not the diodes being too slow for proper rectification (obviously, rectification works just fine). The problem occurs when these spikes couple into some sensitive circuitry. This is difficult to avoid, as they appear as common mode on the transformer wires.

ANOTHER EDIT

When the oscilloscope disagrees with the simulator, one or both could be wrong, however it always helps to model the real circuit (ie, account for transformer inductance) and watch the sim parameters...

enter image description here

enter image description here

This works as expected. Due to transformer inductance (current lags voltage), the diode turns off a little bit later than what would be expected from visual comparison of transformer unloaded voltage (black) and capacitor voltage (green). A perfect diode would also turn off at the same moment, then the transformer secondary voltage would snap back down to its unloaded value. This is normal.

What recovery adds is a tiny amount of time for diode current to turn negative. Thus, when the diode blocks, inductor current is not zero, rather it is a few mA. This is not a lot, because 50Hz is very slow.

However, when the diode turns off, the inductor is large enough to produce a sharp negative voltage spike which causes ringing in the LC tank formed by the inductance and the diode's capacitance, which is an EMI problem.

In real life, the ringing is much shorter than shown here, because the inductor has lots of losses at high frequency. Here it rings at about 1MHz.

Using faster diodes (low Qrr) makes them turn off at a lower negative current, so it reduces the amount of energy available to excite the ringing. Soft recovery diodes produce a smoother current step, which has the same effect. So, fast/soft recovery diodes work to reduce EMI problems here. But a cheaper fix is to just put caps across the diodes. It works just as well.

enter image description here

Red trace is without caps and without snubber. It rings at 1MHz. Adding 10nF cap across the diode lowers ringing frequency to 100kHz (green) which is no longer a problem, it also smoothes the edges, so the EMI problem is gone. Blue is with snubber added (R3/C3). Much cleaner, but not strictly necessary. Transformer iron losses would mostly dampen it anyway.

enter image description here

Summary: Superfast diodes cause less noise, but it's only because of a subtle side-effect: they let less current (and energy) build up in the inductor before turning off, at which point inductor stored energy is turned into ringing. Absorbing the inductor energy in a capacitor and dissipating it in a snubber resistor is just as good, in fact it works better for less money... which means there is no real cost/benefit gain for expensive superfast diodes. But they work. They're just not the optimum solution.

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    \$\begingroup\$ Once the rectified output of the bridge rectifier meets the filter capacitors, what difference does subtle charitaristics of different diodes make? \$\endgroup\$ – Mike Waters Aug 12 '17 at 3:30
  • \$\begingroup\$ A diode snapping shut has very high di/dt, it emits pulsed wideband RF. A big smoothing cap has >20nH inductance plus trace inductance and doesnt filter anything at RF. Plus the transformer wiring and thru hole rectifier bridge act as loop antennas. Putting caps right across the diodes reduces the area of the loop antenna, making it much less efficient at radiating trash. Layout is important, caps should be right across the diodes. \$\endgroup\$ – peufeu Aug 12 '17 at 7:53
  • \$\begingroup\$ Thanks for this - the speed of the diodes directly answered my question as to the benefits of using separates diodes. Cheers - I have plenty of reading to do! \$\endgroup\$ – abza Aug 12 '17 at 19:47
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    \$\begingroup\$ This is a completely erroneous solution description for a rectifier operating at 50/60 Hz. To actually need fast recovery diodes you need to have a fast dV/dt rate for the inbound signal. For a sine wave being rectified the rate of change is 0 when the signal is at it's peak. With the model of the diode being Vf and a series resistor, and the output being a capacitor storage. The forward current actually tails off slowly. There is no fast transient voltage to require a fast switching diode. \$\endgroup\$ – Jack Creasey Aug 13 '17 at 2:06
  • \$\begingroup\$ Diode do not stop conducting at the top of the sine wave, but a little bit later, unless the load draws zero current. See edits. \$\endgroup\$ – peufeu Aug 13 '17 at 10:16
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Almost invariably the type of bridge rectifier you show is not cheaper than individual diodes and contains the same diodes you might use in a discrete bridge. The molded units are:
1. Typically a single screw mounting to make physical assembly where there is no PCB easier.
2. Easier to mount on a heatsink when in an Aluminum case (the larger sizes) and you can have Tab connections for easy physical wiring. 3. Typically for use below 400 Hz

The TO220 and the like will contain wire bonded and unencapsulated discrete diodes. These form factors are much easier to handle (both human and machine assembly)

The MUR860 is NOT a bridge rectifier however and would be unlikely to be used in the same applications you see molded bridge rectifiers used. This is a high speed diode pair used in switching power supplies and a relatively specialized device.

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    \$\begingroup\$ Ah, but this is the world of "audiophiles" where often normal rules and common sense do not apply. Some people pay north of $1000 for AC line cords because they are supposed to make your amplifier sound better. Maybe they have oxygen-free copper. Ditto for speaker wires. If I remember right someone had a special brick you could put on your amplifier that would improve the sound quality as well. (Of course there are sensible audiophiles, it's just that there's a lot of misinformation and a lot of scam artists.) \$\endgroup\$ – John D Aug 11 '17 at 15:55
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    \$\begingroup\$ @JohnD. There is no accounting for the audiophile ....that's why you see products such as Monster cables at extortionate prices being used. Sad really. \$\endgroup\$ – Jack Creasey Aug 11 '17 at 16:03
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    \$\begingroup\$ @johnD - 1000 USD is nothing. There are people spending 10,000 USD getting separate utility poles installed: wsj.com/articles/… \$\endgroup\$ – Whiskeyjack Aug 12 '17 at 8:41
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    \$\begingroup\$ @Whiskeyjack Amazing. Do you think the recording studio that made the record the guy was listening to paid for their own pole to power all the recording and mastering equipment, LOL? \$\endgroup\$ – John D Aug 12 '17 at 9:27
  • \$\begingroup\$ @johnD - I seriously want to secretly change their setup to regular wires and utility poles and see if they feel any difference. And if not, disclose it to them and see their reaction. \$\endgroup\$ – Whiskeyjack Aug 12 '17 at 10:28
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When looking at the performance of rectifiers operating at 50/60 Hz you can use the CircuitLab circuit simulator.

Here is a simple half wave rectifier using a 1N4001 diode. This has a very poor reverse recovery time, but it is inconsequential at 50/60 Hz. I've added some series resistance to the AC source since in this simulator it's not part of the source element.

schematic

simulate this circuit – Schematic created using CircuitLab

If you run the simulation you will see that there is no reverse recovery current seen. This is because at 50/60 Hz the rate of change of the voltage source is very low so any energy stored in the junction is dissipated easily.

enter image description here

The story changes if you raise the frequency however, and at just 1 kHz the reverse recovery time does become a factor. If you examine the curves you'll see that the I(RR) is about 130 mA.

enter image description here

If we go even further to 20 kHz, you can see that the diode is seriously compromised by both junction charge storage and reverse recovery time.

enter image description here

So while Reverse recovery times is a serious issue at high frequencies, at 50/60 Hz they are not. This is primarily because the rate of change in voltage (dv/dt) is very much lower at low frequencies.

Could you put fast recovery diodes into a 50/60 Hz rectifier application, sure you could. Would you see any improvement .....very very doubtful.

I'd challenge anyone to find a good reason to use fast diodes in this type of application.

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  • \$\begingroup\$ Simple. When simulation shows wildly different results from an oscilloscope, usually simulation is wrong ;) Most likely explanations are: the rectifiers in the amp I tested could be slower than 1N4001 (after all they're almost 40 years old technology), and you forgot transformer inductance. I ran your sim again with R1=0R2 with 100µH in series, R2=120R, 1µs timestep (very important), 50Hz, and got the spike as expected. It's an EMI problem, which may or may not matter depending on what nearby circuits can pick the noise up. \$\endgroup\$ – peufeu Aug 14 '17 at 11:14
  • \$\begingroup\$ @peufeu. But simulation shows exactly the results you'd see on a CRO. You keep discussing about when a diode 'snaps off' and the like. They don't, they are not active devices. The only time you can expect transient behavior is when driven by a square wave with very large dv/dt edges. Change the simulation to a square wave to see what I mean. You are giving diodes characteristics they don't have based on misinterpreting (or measuring) signals. But as always, you can have your opinion. \$\endgroup\$ – Jack Creasey Aug 14 '17 at 15:39
  • \$\begingroup\$ Hmm, maybe I explained it wrong. I made another attempt, check it out LOL \$\endgroup\$ – peufeu Aug 14 '17 at 18:45

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